The present invention relates to a method of controlling a gear-shifting process in a power-shift transmission of a vehicle, wherein the power-shift transmission is coupled to an engine and uses a frictional slip clutch as a power-shift clutch.
To change gears in known manually shifted transmissions, e.g., for passenger cars, it is necessary to interrupt the flow of tractive force from the combustion engine to the drive train of the vehicle, e.g., by disengaging a start-up element such as a friction clutch, where the disengagement can he performed either by the driver of the vehicle or by an actuator. The time gap in vehicle traction is noticeable for the user of the vehicle at every gear change. The interruption in traction occurs also in automated transmissions in which the gear-shifting process is performed by means of an actuator. While there are known transmissions that can be shifted while traction is maintained (also known as power-shift transmission s), the state-of-the-art units are primarily automatic transmissions of a planetary-gear design. Although they are practically proven, they are complex and therefore expensive.
To make the gear-shifting process in vehicles more comfortable, the known state of the art also offers a power-shift transmission in the form of a so-called interruption-free gear-shift transmission. This interruption-free transmission, like conventional transmissions, includes a start-up element such as a friction clutch, but in addition, a frictional slip clutch on either the input or output shaft of the transmission is arranged in a such a manner that an engine torque acting on the transmission-input shaft in the engaged condition of the start-up clutch is transmitted through the frictional slip clutch to the transmission-output shaft, whereby it becomes possible to transmit torque also while a gear-change of the transmission is in process. In other words, the flow of traction from the engine to the drive train of the vehicle is maintained during the gear change.
It has been found that the increase in vehicle comfort achievable with an interruption-free transmission of this kind depends to a great extent on the characteristics of the additional frictional slip clutch, which will be referred to as power-shift clutch. Especially when shifting gears under full traction, the heat load on the power-shift clutch is very high, because to a large extent, a transmission of this kind has to be able to transmit the entire engine torque to the drive train of the transmission, if an interruption in traction is to be avoided. Otherwise, a time-gap in the tractive force could be noticed by the driver of the vehicle during a phase of full acceleration when the full amount of driving torque is delivered by the engine.
The heat load on the power-shift clutch becomes greater if the power-shift clutch is arranged to be effective with a higher gear of the power-shift transmission. For example, if the power-shift transmission has five forward gears, the heat load on the power-shift clutch is greater if the power-shift clutch is arranged to be effective with the fifth gear of the transmission than if it were arranged to act on the fourth gear. However, it is desirable in a transmission of this kind that the power-shift clutch be effective in fifth gear because otherwise a shift from fourth into fifth gear could no longer be performed under traction and would therefore be accompanied by an interruption of the tractive force. However, in order to achieve a high degree of durability of a power-shift clutch that is configured as a frictional slip clutch, it is necessary to avoid a thermal overload of the power-shift clutch.
The present invention therefore aims to provide a method of controlling a gear-changing process of a power-shift transmission, whereby the heat load on the power-shift clutch is reduced in order to make the power-shift transmission more durable.
The invention is based on the observation that the gear-shifting process in a power-shift transmission is influenced to a high degree by the clutch torque that is transmitted through the power-shift clutch. The clutch torque affects all of the following: a) the amount of output torque delivered to the drive train of the vehicle during the gear-shifting process, b) the amount of time needed for synchronization and thus the time required for the gear-shifting process, c) the amount of heat generated in the power-shift clutch due to the slipping engagement, and d) the amount of power transmitted through the power-shift clutch. Although it is possible, in principle, to influence the output torque during the entire gear-shifting process by way of the clutch torque flowing through the power-shift clutch, a process of controlling the output torque by way of the clutch torque leads to a large accumulation of energy in the power-shift clutch in the form of heat that is generated by the frictional slip of the power-shift clutch. Albeit that in a power-shift clutch designed as a frictional slip clutch with two or more friction linings or as a laminar-disc clutch with a plurality of friction linings, the heat can be carried away by convection, radiation, or heat conduction into the pressure plate and the disc or discs of the clutch, the power-shift clutch is nevertheless subjected to a high heat load if power-shift processes are performed in rapid succession so that no effective temperature reduction of the power-shift clutch can take place between the individual power-shifts.
The present invention solves the foregoing problem by providing an advantageous method of controlling the gear-changing process, whereby the amount of energy introduced into the power-shift clutch can be lessened. The method also has a favorable effect on the fuel consumption of a vehicle equipped with a power-shift transmission, but has no negative consequences on the desired shifting comfort in the interruption-free power-shift processes.
Under the method according to the invention, a gear-changing process in a power-shift transmission coupled to an engine and equipped with a frictional slip clutch works in a manner where the engine torque as well as the clutch torque are being varied in such a way that the amount of heat introduced into the frictional slip clutch is lowered and, consequently, the temperature increase of the frictional slip clutch is reduced. In other words, in the gear-changing process that is performed as a power-shift process, the inventive method provides that the engine torque as well as the clutch torque are being varied and that the variation is performed in a manner where the amount of energy introduced into the power-shift clutch and converted into heat is reduced in comparison to a gear-changing process where only the clutch torque of the power-shift clutch is being varied and the torque produced by the engine remains, at least to a large extent, constant during the gear-changing process.
In a first phase of the gear-shifting process according to the inventive method, the torque entering the drive train of the vehicle is reduced by lowering the engine torque while the gear clutch of the current gear level is still engaged. Thus, at the beginning of a power-shift process, the torque level is lowered at first by an intervention of the engine control system rather than by using the power-shift clutch. The power-shift clutch is therefore in the disengaged condition and not transmitting any torque at the beginning of the shift process. Because the power-shift clutch uses a slipping engagement to transmit torque between the input shaft and the output shaft of the power-shift transmission, a transmission of torque through the power-shift clutch will cause the latter to heat up. However, at the beginning of the power-shift process according to the inventive method, the output torque is lowered by controlling the engine torque, so that there is no load on the power-shift clutch, and thus no energy is introduced in the power-shift clutch, during the corresponding time phase of the power-shift process.
As a next step after the first reduction of the output torque that has just been described, the power-shift clutch is brought into engagement, so that a clutch torque will flow through the power-shift clutch. The purpose of the power-shift clutch is to prevent an interruption in the flow of tractive force from the engine to the drive train of the vehicle during the gear-shifting process. The clutch torque transmitted by the power-shift clutch cannot be increased abruptly, because the comfort level of the shifting process is determined by the time gradients of the clutch torque. Consequently, the engagement of the power-shift clutch, and thus the magnitude of the clutch torque, is increased gradually during this second step. In the course of this process, the reduction of the output torque is continued, while the engine torque is simultaneously raised again. The increase in engine torque, e.g., to the maximum torque level of which the engine is capable, has the result that with full acceleration of the vehicle under maximum engine torque or also with a lower amount of engine torque, the driver of the vehicle will experience no interruption in traction when the currently used gear-stage is shifted out of engagement. Because the clutch torque during this phase of the gear-changing process is raised to a level that is at least equal to the engine torque, the slippage between the friction surfaces of the power-shift clutch is relatively high. It is therefore advantageous if the time needed for equalizing the clutch torque and the engine torque is kept short. In accordance with a further developed version of the method, the gear clutch of the current gear stage can be taken out of engagement as soon as the engine torque and the clutch torque have been found to be substantially equal.
To allow the gear clutch to be retracted from engagement as rapidly as possible, it is advantageous if an actuator applies a retracting force to the gear clutch already before the engine torque and the clutch torque have become substantially equal, so that the currently used gear stage can be disengaged as soon as the torque passing through the gear clutch of the currently used gear has fallen below a preset threshold level, whereby the time for the disengaging process can be kept short. The disengaging process can be monitored, e.g., by observing the number of revolutions of an actuator motor.
After the disengaging process has been completed and prior to bringing the new gear stage into engagement, the process of synchronization takes place, during which the output torque is determined exclusively by the clutch torque transmitted through the power-shift clutch. It has therefore proven to be advantageous if, in accordance with the inventive method, the engine torque is lowered for the synchronization and the clutch torque is adjusted in such a way that the output torque corresponds to an output torque level that will be present after the gear-shifting process.
Because during the synchronization phase, the previously used gear stage has already ceased to transmit torque and the new gear stage to be engaged is not yet transmitting torque, the output torque during the synchronization phase is determined by the clutch torque that flows through the power-shift clutch. To shorten the synchronization time, it is therefore of advantage if the engine torque is lowered to a minimum level as rapidly as the engine control system will permit.
Therefore, to shorten the synchronization time, a further developed version of the inventive method provides that after the previously used gear stage has been disengaged, the engine is set to a drag-torque level, to shorten the time during which the engine continues to run under its own momentum. As the engine is set into the drag mode, the rpm rate of the engine and transmission input shaft is lowered and the synchronization process is accelerated, whereby the amount of energy accumulated in the power-shift clutch during the synchronization phase is reduced. This approach, whereby the engine is set into the drag mode as rapidly as possible, requires an engine with an appropriately fast response. With a combustion engine that does not allow a rapid change into the drag mode, it is advantageous for the purpose of shortening the synchronization time if the clutch torque is increased above the maximum engine torque, so that the rpm rate of the engine and the input shaft of the transmission is lowered. The increased clutch torque actively decelerates the engine against the drive train, so that by increasing the clutch torque above the maximum engine torque, the synchronization time is shortened and the accumulation of energy taking place in the power-shift clutch during the synchronization phase is again reduced.
According to a further developed version of the method, after the synchronization has essentially been completed, the clutch torque and the engine torque are being set to substantially equal levels. The gear clutch of the new gear is brought into engagement at a point when the rpm rate and acceleration of the input shaft and output shaft of the power-shift transmission are substantially equal. During this engagement phase, the slippage-rpm rate of the power-shift clutch is low, and the clutch torque corresponds, at least to a large extent, to the engine torque, so that only a small amount of energy is introduced into the power-shift clutch. When the gear clutch of the new gear stage has come into engagement, the power-shift clutch is removed from engagement and the clutch torque is thereby lowered. The disengagement of the power-shift clutch has to be performed according to a time gradient that agrees with specified requirements of driving comfort, as the power-shift clutch continues to transmit torque and, e.g., an abrupt disengagement of the power-shift clutch could cause a discontinuity of the output torque in the drive train of the vehicle, which would be felt by the occupants as a jolt.
The inventive method further provides that the temperature of the frictional slip clutch be monitored in order to avoid a heat overload. During a power-shift, the friction-based transmission of torque causes an energy influx and conversion of energy into heat in the power-shift clutch. At least part of the heat is carried away by convection, radiation, or heat conduction into the clutch plate and the disc, but if a series of power-shifts is performed in short time intervals, the temperature of the power-shift clutch will rise because there is not enough time for the heat to escape. A thermal overload can cause mechanical damage to the power-shift clutch.
The invention therefore provides that, when a temperature limit of the frictional slip clutch has been found to be exceeded, the clutch torque is lowered in function of the temperature. The reduced clutch torque will still allow a power-shift to be performed but will cause a reduction in the comfort level of the shift process. This condition of the vehicle that is equipped with the power-shift transmission is also communicated to the driver by way of the decreased comfort level because, as a rule, an excessive temperature of the power-shift clutch occurs only if the driver is not operating the vehicle in conformance with prescribed procedure. For example, if the driver uses the gas pedal in a way that causes a rapid succession of up- and down-shifts in the lower gears at full traction and high engine-rpm rate, the decrease in the level of driving comfort will alert him or her of their unsound style of driving.
In accordance with a further developed version of the method, the power-shift transmission is being prevented from down-shifting into first gear, if the frictional slip clutch temperature has been found to be too high. Because of the large step between the transmission ratios of first and second gear, a power-shift from first into second gear is especially prone to cause a high thermal load on the power-shift clutch. A further temperature rise of the power-shift clutch due to power-shifting under traction from first into second gear is therefore avoided by the measure of blocking the down-shift into first gear.
The inventive method further provides that, if the friction-clutch temperature has been found to be too high, the frictional slip clutch is kept disengaged during the synchronization phase, so that no torque is transmitted. While this measure will noticeably reduce the comfort level of the shifting process because of the total interruption of the tractive force, it prevents damage that would be caused to the power-shift clutch by a temperature that has been found to exceed the permissible level by a clear margin. This measure is proposed in light of the fact that a temperature of the required excessive level cannot occur if the vehicle with the power-shift transmission is used according to prescribed procedure, but will only be caused by an incorrect behavior of the driver who will experience a complete loss of traction during the shift process and thereby be alerted to his or her inappropriate style of driving.
Another further developed version of the inventive method provides that, after the frictional slip clutch has been found to be too hot, preset rpm-rates for the gear changes of the power-shift transmission are lowered in order to reduce the amount of energy that is introduced into the power-shift clutch. It needs to be mentioned that the foregoing measures that are taken after finding an excessive temperature of the frictional slip clutch can be applied alternatively or also in combination, dependent on the extent to which the temperature exceeds a permissible level.
The invention further provides the measure of monitoring the power transmitted through the frictional slip clutch and lowering the clutch torque, e.g., to the level of the maximum engine torque, if a preset maximum power level has been found to be exceeded so that, for example during the synchronization phase, the clutch torque is no longer being raised above the magnitude of the engine torque, whereby damage to the power-shift clutch can be safely prevented. This measure allows to avoid damage to the power-shift clutch even before an excessive temperature of the power-shift clutch can be registered.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.